1. Field of the Invention
The present invention is directed to improved aqueous polyisocyanate dispersions having a reduced isocyanate content and to their use as binders for fibers, especially fiberglass.
2. Description of the Prior Art
It is known from U.S. Pat. No. 4,904,522 to use aqueous dispersions of polyisocyanates as binders for fiberglass. When compared to known phenol/formaldehyde resins, the polyisocyanate binders cure at a much lower temperature, do not split off volatile monomers, provide at least the same strength, are not a potential formaldehyde source and do not require an amino alkoxy silane adhesion promoter. However, the polyisocyanate binders disclosed in the copending application have relatively high quantities of unmodified monomeric diisocyanates. Because the presence of monomeric diisocyanates may lead to industrial hygiene problems, it would be beneficial to reduce the content of monomeric diisocyanates as much as possible.
Another deficiency of the polyisocyanate binders disclosed in U.S. Pat. No. 4,904,522 is that it is difficult to achieve complete cure during subsequent heating of the fiberglass mats in the oven zone. During the production of fiberglass mats, the fibers are treated with the aqueous polyisocyanate binders and continuously run through an oven zone in order to evaporate water and to cure the polyisocyanate resins.
When aqueously dispersed polyisocyanates are used as binders, the water serves as the co-reactant for the isocyanate groups to form polyureas. If the isocyanate content of the polyisocyanate binder is too high, water is evaporated in the oven zone before the reaction is complete and as a result, an uncured, unusable fiberglass mat is obtained.
A further disadvantage is that even though the preferred polyisocyanates of U.S. Pat. No. 4,904,522, i.e., polyphenyl polymethylene polyisocyanates, exhibit a low vapor pressure at ambient temperature, they still contain high amounts (as much as 70% by weight) of monomeric diphenyl methane diisocyanates. Upon exposure to the high temperatures in the oven zone these monomeric diisocyanates can be volatilized which results in high concentrations in the exhaust gases. This represents an environmental hazard if these exhaust gases escape into the atmosphere of the workplace or the air surrounding the manufacturing facility.
One method for lowering the isocyanate content of the polyisocyanate binders would be to react the polyisocyanates with polyols to form isocyanate-terminated prepolymers prior to dispersing in water. However, this results in products which have high viscosities at the desired low isocyanate content, i.e., an isocyanate content of less than 10% by weight, based on solids, and thus are too viscous to disperse in water even if they have been hydrophilically modified. Another method of avoiding uncured polyisocyanate binders is to incorporate catalysts which promote the isocyanate/water reaction in the aqueously dispersed polyisocyanate binders. However, this method also does not result in a complete cure of the polyisocyanate prior to evaporation of water in the oven zone.
One method of reducing the isocyanate content of the aqueous polyisocyanate binders is disclosed in copending application, U.S. Ser. No. 07/529,056, filed May 25, 1990. In accordance with this application the aqueous polyisocyanate binders are reacted with primary or secondary monoamines containing at least one hydroxyl group in an amount sufficient to provide an equivalent ratio of amino groups to isocyanate groups of 0.6:1.0 to 0.95:1.0. The disadvantage of this process is that the reaction of even 20 to 30% of the isocyanate groups of aqueous polyisocyanate binders having a solids content of 10% by weight with the amino groups from ethanolamine results in dispersions which become so viscous and structured that they no longer flow like liquids and cannot be readily reliquified.
A second method of reducing the isocyanate content of aqueous polyisoycanate binders and increasing their storage stability is disclosed in copending application, Ser. No. 677,011. In accordance with this application the aqueous polyisoycanate binders are reacted with low molecular weight polyamines in order to encapsulate the polyisocyanate and render it less reactive with water. While the process disclosed in this application is effective for increasing the storage stability, it is only possible to react about 30 to 40% of the isocyanate groups of the polyisocyanate. The remainder of the isocyanate groups are present in the interior of the dispersed polyisocyanate particles and do not react with the polyamine.
Accordingly, it is an object of the present invention to provide aqueously dispersed polyisocyanate binders which have reduced isocyanate contents and do not possess any of the deficiencies of the previously described binders. It is an additional object to provide aqueously dispersed polyisocyanate binders which need only small amounts of water to cure and which develop their final properties much more rapidly under the given conditions. It is a further object of the present invention to provide aqueously dispersed polyisocyanates that despite their low isocyanate contents still exhibit film forming properties and excellent adhesion to the fibers. Finally, it is an object of the present invention to provide aqueously dispersed polyisocyanate binders which have the advantage of containing only small amounts of volatile monomeric diisocyanates.
These objects may be achieved in accordance with the present invention as described hereinafter.